Formation of Thermally Stable Bulk Heterojunction by Reducing the Polymer and Fullerene Intermixing

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Bulk heterojunction (BHJ) morphology is crucial for organic photovoltaic (OPV) performance.
• Intermixing in BHJ active layers can limit device stability and efficiency.
• Developing processing techniques for controlled BHJ nanostructure is essential for efficient and stable OPVs.

Purpose of the Study:

• To develop a morphologically stable BHJ with a large heterojunction area using sequential deposition (SqD).
• To investigate the effect of ternary solvents on nanostructured polymer layer formation.
• To evaluate the power conversion efficiency (PCE) and thermal stability of SqD-processed OPVs compared to blended solution deposition (BSD).

Main Methods:

• Sequential deposition (SqD) of nanostructured PTB7 polymer and fullerene layers.
• Preparation of nanostructured PTB7 using a ternary solvent (chlorobenzene, 1,8-diiodooctane (DIO), 1-chloronaphthalene (1-CN)).
• Characterization using grazing incidence X-ray diffraction (GIXRD) and thermal annealing tests.

Main Results:

• SqD resulted in a BHJ with reduced polymer-fullerene intermixing compared to BSD.
• SqD-processed OPVs (SqD-OPV) achieved a PCE of 7.43%, comparable to BSD-processed OPVs (BSD-OPV).
• SqD-OPV demonstrated excellent thermal stability, maintaining initial PCE after 10 days at 140°C, while BSD-OPV retained only 78% efficiency.

Conclusions:

• Sequential deposition of nanostructured layers offers a viable strategy for creating morphologically stable BHJs in OPVs.
• The use of ternary solvents promotes polymer ordering, enabling nanostructure formation for improved BHJ properties.
• SqD processing significantly enhances the thermal stability of organic photovoltaics, crucial for practical applications.